1. Field of the Invention
This invention relates to a cell type oxygen sensor for measuring oxygen concentration in exhaust gas from an internal combustion engine such as the oxygen concentration after chemical equilibrium is reached in the exhaust gas.
2. Description of the Prior Art
A typical oxygen sensor for exhaust gas in the prior art which is used at the present comprises a solid electrolyte tube with a bottom at an end, an inner electrode coated on the inner surface of said electrolyte tube so as to contact with fresh air as reference gas, an outer electrode coated on the outer surface of said electrolyte tube so as to contact with exhaust gas, a metallic housing tube holding said solid electrolyte tube while allowing exposure of said outer surface at the bottom thereof to the exhaust gas, a protective means having a pipe coaxially extending from one end of the housing tube so as to surround the bottom side of said solid electrolyte tube, and a gas passage defined in said pipe for restricting direct collision of the exhaust gas with the outer surface of said solid electrolyte tube. The protective means prevents impulsive hot exhaust gas stream and fine particle in the exhaust gas from directly striking and colliding with the porous membrane type electrode layer coated on the outer surface of said solid electrolyte tube, so as to avoid comparatively quick deterioration of the performance of the electrode.
One typical form of the conventional protective means includes a pair of metallic pipes with different diameters, each of said pipes having a plurality of holes bored through the sidewall thereof, which metallic pipes are coaxially connected while offsetting the holes of one pipe from the holes of the other pipe as shown in U.S. Pat. No. 4,065,372. The protective means of such two-pipe type has shortcomings in that the thermal mass of the protective means tends to become large, despite that a large thermal mass is against the demand for a low activating temperature (i.e. operation-starting temperature) of the exhaust gas sensor, and that the protective means is rather costly. Another form of the conventional protective means consists of one metallic pipe whose sidewall is severed along the long line extending in the axial direction thereof, and a plurality of long but narrow holes are formed with spacings in the circumferential direction of the pipe by bending the sidewall inwardly along one side of each of said long lines so as to make openings substantially in the tangential directions. The protective means of the one-pipe type has shortcomings in that the narrowness of the holes tends to increase resistance against gas flow therethrough despite the comparatively large length thereof resulting in a poor efficiency of exchanging gas, and that such narrow holes are difficult to make wider because if one tries to bend the sidewall deeper toward the inside of the pipe for broadening the openings, the axially long sidewall portions between the adjacent holes are apt to be deformed to make the bending work difficult.